The present invention relates generally to conical horn antennas used in commercial communications systems, including satellite communications, and more specifically to a conical horn antenna that yields performance substantially equivalent to, or even greater than, that of a corresponding corrugated horn antenna.
The ability of a horn antenna to produce the proper primary radiation pattern is determined by its size, shape, and internal structure, the latter interacting with the microwave energy. Other important factors are isolation as determined by cross-polarization purity, and also side-lobe performance. It is known in the art that for the operation of transmission/reception antennas having emission and two orthogonal polarizations, it is necessary to employ antennas preferably having low cross-polarization. A rotational-symmetrical radiation field and a low reflection factor are presumed. Corrugated horn antennas are especially used when there is a need for low cross-polarization and possible low side lobes across a large frequency range, for example, as a feeding element in reflector antennas or as an individual antenna element operating in the micro or millimeter-wave ranges.
Corrugated horn antennas, as illustrated in FIG. 1, are typically produced by electroforming an exterior surface 100, onto a reverse mold, called a mandrel. The mandrel is first machined to the tolerances required for the structure of the corrugations 110 of the horn. After electroplating, the exterior is machined to the desired shape, and the mandrel, which has a lower melting point than the antenna material, is melted out. Corrugated horn antennas, however, have been difficult to produce commercially, especially in the millimeter-wave range. This is due to the high production costs arising from the complicated structural characteristics and extreme accuracy that is required in machining the mandrel.
As a result, conventional conical type horn antennas often must be used even though the electrical characteristics are substantially below those of the corrugated type horn antenna. The conventional conical horn is very simple, easy to fabricate, and low in cost. However, the rotational symmetry is not perfect and the cross polarization level is in the range of xe2x88x9219 dB, as opposed to the desired low cross polarization level of xe2x88x9230 dB. The unequal E and H plane patterns in a conventional horn operating on its dominant transverse electrical (TE11) mode result from different boundary conditions at the top and side parts of the circular waveguide. To correct this asymmetry, the same aperture field distribution at the E and H planes must be created. One method of correcting this asymmetry is by installing short-circuited quarter-wavelength grooves on the walls of the horn. Another method is by covering the horn walls with another type of impedance structure, such as a pure dielectric or a dielectric which has an impedance structure printed on it
Other corrugated horn antennas are known in the art where the horn wall is made to be anisotropic and reactive, and it complies with the balanced hybrid condition of the hybrid HE11 mode within the desired frequency band. Thus, the diagrams of radiation in the E and H planes will become almost alike and give low cross-polarization.
Even though this type of antenna has in principle, satisfactory characteristics, again, it is burdened with disadvantages with respect to production costs.
It is an object of this invention to create a horn antenna that has good electrical properties, substantially equivalent to, or better than, those of the corrugated type horn antenna, and is as easy, simple, and inexpensive to manufacture as the conical type horn antenna.
It is a further object of this invention to provide a conical horn without corrugations that permits an increase in radiated power above that of a conventional corrugated horn without electrical break down.
It is a further object of this invention to provide a broadband horn (8 to 18 GHz) which has a return loss of less than 30 dB in the frequency range of 10.7 GHz to 18 GHz.
In the present invention, a horn antenna, positioned with respect to a central axis, comprises a conical horn portion having two ends, one of the ends of greater diameter than the other end, the conical horn portion having a smooth interior wall without corrugations defining a cavity, the end of greater diameter defining an interfacing end. An output structure is positioned at the interfacing end of the conical portion, with the output structure having at least one dielectric region such that the flow of electrical current through the wall in the direction along the central axis is substantially impeded. The dielectric region preferably comprises vacuum, air, free space, or a dielectric material with very low losses. The impedance output structure is positioned at the interfacing end as a separate structure or is positioned at the interfacing end as a unitary construction with the conical portion.
In a different aspect of the embodiment, the conical horn portion can comprise at the end of lesser diameter a circular waveguide with a smooth interior wall without corrugations. The conical horn portion can further comprise a flare break portion at the end of greater diameter of the conical horn portion. The flare break portion has a smooth interior wall without corrugations, and has an aperture defining the interfacing end.
In one embodiment of the horn antenna, the impedance output structure further comprises at least one ring offset from the interfacing end of the conical portion by at least one post defining the at least one dielectric region between the interfacing end and the at least one ring. In a variation of the embodiment, the dielectric region further comprises a dielectric material disposed in at least a portion of the dielectric region.
In another embodiment of the horn antenna of the present invention, the impedance output structure is a helical coil winding, the turn of the winding forming at least one dielectric region between the winding and the interfacing end of the conical horn portion of the horn antenna. In a variation of the embodiment, the helical coil winding further comprises a dielectric jacket surrounding the winding.
In still another embodiment of the present invention, an array antenna comprises a plurality of horn antennas positioned with respect to a central axis, where at least one of the plurality of horn antennas comprises a conical horn portion having a wall defining an interior cavity and an interfacing end. An output structure is positioned at the interfacing end of the conical horn portion, the output structure having at least one dielectric region such that the flow of electrical current through the wall in the direction of the central axis is substantially impeded.
Due to the presence of a dielectric region between each ring or between the turns of the helical coil winding, the electrical current in the direction of the central axis that is induced by the tangential component of the magnetic field perpendicular to the axis is substantially prevented from flowing through the wall of the horn antenna, therein causing the cross-polarization level to decrease to the range of 27-35 dB.